As the need for electricity continues to grow, the need for higher capacity transmission and distribution lines grows as well. The amount of power a transmission line can deliver is dependent on the current-carrying capacity (ampacity) of the line. The ampacity of a line is limited by the maximum safe operating temperature of the bare conductor that carries the current. Exceeding this temperature can result in damage to the conductor or the accessories of the line. Moreover, the conductor gets heated by Ohmic losses and solar heat and it gets cooled by conduction, convection and radiation. The amount of heat generated due to Ohmic losses depends on current (I) passing through it and its electrical resistance (R) by the relationship Ohmic losses=I2R. Electrical resistance (R) itself is dependent on temperature. Higher current and temperature leads to higher electrical resistance, which, in turn, leads to more electrical losses in the conductor.
Several solutions have been proposed in the art. WO 2007/034248 to Simic discloses overhead conductors coated with a spectrally selective surface coating. The coating has a coefficient of heat emission (E) higher than 0.7 and coefficient of solar absorption (A) that is less than 0.3. Simic also requires that the surface be white in color to have low solar absorption.
DE 3824608 discloses an overhead cable having a black paint coating with an emissivity greater than 0.6, preferably greater than 0.9. The paint is made of a plastic (e.g. polyurethane) and black color pigment.
FR 2971617 discloses an electric conductor coated with a polymeric layer whose emissivity coefficient is 0.7 or more and solar absorption coefficient is 0.3 or less. The polymeric layer is produced from polyvinylidene fluoride (PVDF) and a white pigment additive.
Both FR 2971617 and WO 2007/034248 require white coatings that are not desirable due to glare and discoloration over time. Both DE 3824608 and FR 2971617 require polymeric coatings that are not desirable due to their questionable heat and wet aging characteristics.
US 20120074122 Al teaches the application of high emissivity coatings, such as those found in U.S. Pat. No. 7,105,047 and U.S. Pat. No. 6,921,431, on or adjacent to heating elements within a baking oven to modify the radiating heat. Those application and patents are incorporated herein by reference.
Therefore, there remains a need for a durable, inorganic, non-white coating for overhead conductors that allow the conductors to operate at reduced temperatures.